The present invention relates to a method and apparatus for distributing a controlled thin layer of particulate batch material over the top surface of a mass of molten, heat softenable material such as glass, such as in a circular electrically heated, open-top furnace and in a furnace for refractory materials in particular.
In the operation of an open-top electrically heated furnace, heat softenable batch materials such as glass forming oxides are continually fed onto the upper surface of the melt to replenish the melt in compensation for the refined molten material that is continuously withdrawn from the furnace through a centrally located outlet. These batch materials when spread in a layer over the upper melt surface also serve as a partially insulating blanket to minimize the heat loss to the ambient through the top of the melt. Problems arise when an even layer of batch of a certain minimal thickness is not maintained over the whole upper surface of the melt. Regions of the batch layer that are too thin "melt out," exposing the molten material and creating regions of excessive heat loss, which occurrence tends to lower melt temperatures thereby lowering melt flow rates. In addition, regions of uncovered melt may cause equipment located over the furnace, such as a batch feeder, to be exposed to excessive heat. When too thick a layer is deposited melt temperature will rise, increasing melt flow rate. Too much batch also promotes formation of a surface crust when hinders the passage of overlaying batch therethrough.
Inability to maintain a uniform thin layer of batch at an optimal thickness contributes to undesirable hot and cold thermal cycling of the melt and a resulting undesirable fluctuating of furnace performance. These ill effects are aggravated as the operating temperature of a furnace is increased; thus in refractory furnaces which typically operate at 3400.degree. F.-4000.degree. F. the need is greatest for the controlled deposit of batch.
One known method of feeding batch material to a circular, open-top furnace includes a distribution pan mounted to move over the melt surface in a circular path to sweep from the inner to the outer regions of the melt surface in an oscillating manner. Batch is fed to the pan which in turn deposits the batch across the melt surface. The pan does not evenly deposit the batch and the operation must be supplemented with manual distribution by an operator using a wooden hoe in an attempt to form a uniform thin layer of batch of desired thickness. The batch feeders disclosed in U.S. Pat. No. 3,980,460 are designed for lateral distribution of batch material over the length of furnace chambers of a square or rectangular form. Such methods and equipment do not deposit a thin uniform layer of batch.
It is an object of the invention to provide in an open-top electric furnace, a batch feeder for depositing batch materials onto the entire melt surface in a uniform thin layer and to continually supplement that layer in a controlled manner that ensures that an optimal thickness will be maintained.
Another object of the invention is to feed batch material to an open-top electrical furnace in a manner that effectively and efficiently reduces heat loss through the upper surface of the melt.